Differential voltage sensing method

ABSTRACT

A differential voltage sensing method for achieving input impedance matching comprises the steps of: providing a first bio-potential signal to a first variable resistor for generating a first signal; providing a second bio-potential signal to a second variable resistor for generating a second signal; differentially amplifying first and second signals for generating a third signal; selecting an operation band of the third signal for generating first and second logic signals; and dynamically adjusting one of the impedances of the first and second variable resistors according to the first and second logic signals, wherein each of the first and second bio-potential signals has a common signal voltage level and a differential signal voltage level.

BACKGROUND

1. Technical Field

This disclosure relates to a differential voltage sensing system and amethod for achieving input impedance matching, and, in particular, to adifferential voltage sensing system and method for sensing bio-potentialsignals.

2. Description of the Related Art

With the increased focus on personal health among people in recentyears, and with the increasing life expectancy of the global population,applications for measuring and monitoring bio-potential signals such asthe electrocardiogram (ECG) signal, the electroencephalogram (EEG)signal, the electromyogram (EMG) signal, and the electrooculogram (EOG)signal are frequently required, and the related products areincreasingly popular. One widely-used bio-potential measurementapparatus measures signals from the human body via external skin patchelectrodes adhesively attached to the subject's skin, and monitorsphysiological conditions of the heart or the brain via recording andfurther analysis.

The bio-potential signal acquired from the human body is quite small,and is easily subject to interference from the external environment orfrom other bio-potential signals from the test subject. Therefore, adifferential voltage sensing system with a high common mode rejectionratio (CMRR) is suitable for measuring the small bio-potential signalsso as to reduce the influence of noise on the measurement of thebio-potential signals. An input signal of the differential voltagesensing system usually has a common mode signal and differential modesignal. The main component of the common mode signal is noise, and thenoise can be canceled by a differential amplifying method because thecommon mode noise signals are substantially equal in amplitude butinvert of phase with one another. Therefore, the differential amplifyingmethod can eliminate the common mode noises and amplify the smallbio-potential signals.

However, the common mode noise signals cannot be eliminated when theinput impedances of the differential voltage sensing system are notequal, and without elimination of the common mode noise signals theelectrical signals are easily subject to distortion because of theinterference noise. For example, skin morphology or adhesion of theelectrode often results in differences of skin-electrode interfaceimpedance so that the input impedances of the differential voltagesensing system are unequal. In particular, effective electrical signalscannot be captured from multi-pole EEG or multi-lead ECG signals whendefective electrodes are present.

U.S. Pat. No. 6,208,888 discloses a voltage sensing system with inputimpedance balancing. The system comprises a feedback controller thatadjusts an effective impedance associated with the second electrodeaccording to a differential mode signal, a common mode signal, and animpedance associated with the first electrode. As a result, signalsassociated with each electrode undergo a similar degree of gainattenuation. However, since the input impedance balancing is achieved byvarying the conversion characteristics of the circuit, an oscillatorynoise is generated by such arrangement.

U.S. Pat. No. 5,233,985 discloses a cardiac pacemaker employing anoperational amplifier output circuit for producing an electricalstimulating pulse. The circuit employs a variable resistor as a load forcapturing signals and finds optimal common mode/differential modematching points for performing signal measurement. However, such methodemploys manual operation to vary input impedances for matching, andcontrols signals by a virtual load.

Accordingly, there is a need to provide a differential voltage sensingsystem and a method for sensing bio-potential signals so that commonmode noise signals can be eliminated using dynamic matching of the inputimpedances.

SUMMARY

The disclosure relates to a differential voltage sensing system andmethod for using the same. According to one exemplary embodiment, thedifferential voltage sensing system comprises a first amplifyingcircuit, first and second variable resistors, a signal capture unit, anda logic control unit. The first variable resistor has an input terminalconnected to a first node, an output terminal connected to a first inputterminal of the first amplifying circuit, and a control terminal. Thesecond variable resistor has an input terminal connected to a secondnode, an output terminal connected to a second input terminal of thefirst amplifying circuit, and a control terminal. The signal captureunit is connected to an output terminal of the first amplifying circuit,and the logic control unit is connected to the signal capture unit, thecontrol terminal of the first variable resistor, and the controlterminal of the second variable resistor. The impedances of the firstand second variable resistors can be adjusted dynamically according tooutput signals of the logic control unit so as to achieve inputimpedance matching.

According to yet another exemplary embodiment, the differential voltagesensing method for achieving input impedance matching comprises thesteps of: providing a first bio-potential signal to a first variableresistor for generating a first signal; providing a second bio-potentialsignal to a second variable resistor for generating a second signal;differentially amplifying first and second signals for generating athird signal; selecting an operation band of the third signal forgenerating first and second logic signals; and dynamically adjusting oneof the impedances of the first and second variable resistors accordingto the first and second logic signals, wherein each of the first andsecond bio-potential signals has a common mode signal voltage level anda differential mode signal voltage level.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the disclosureand, together with the description, serve to explain the principles ofthe disclosure.

FIG. 1 is a block diagram of a differential voltage sensing system inaccordance with an exemplary embodiment;

FIG. 2 is a flowchart illustrating an exemplary embodiment of adifferential voltage sensing method;

FIG. 3 is a block diagram of a differential voltage sensing system inaccordance with another exemplary embodiment; and

FIG. 4 is a block diagram of a differential voltage sensing system inaccordance with another exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a differential voltage sensing system 10 inaccordance with an exemplary embodiment. The differential voltagesensing system 10 comprises first and second electrode interfaces 11 and12, first and second variable resistors VCR₁, VCR₂, first and secondamplifying circuits OP₁ and OP₂, a signal capture unit 14, and a logiccontrol unit 16. As shown in FIG. 1, the first electrode interface 11 isconnected between an input node in₁ and a node N₁, while the secondelectrode interface 12 is connected between an input node in₂ and a nodeN₂, wherein the input nodes in₁ and in₂ are connected to two differentpositions of a single bio-potential source. The first and secondelectrode interfaces 11 and 12 are configured to detect bio-potentialsignals, such as ECG, EOG, EMG, or EOG signals, from a test subject,wherein these bio-potential signals have a common mode signal voltagelevel and a differential mode signal voltage level. Each of theelectrode interfaces 11 and 12 has an effective skin-electrode impedancemodeled by an interface resistor R, in parallel with an interfacecapacitor C_(i). In addition, the impedances of the electrodes can bevaried widely. For example, when the test subject is engaged in physicalactivity, perspiration may result in reduced resistance due to skinmoisture. Alternatively, the resistance may increase due to poorconnection or poor adhesion caused by dry electrodes, dry skin, or otherskin characteristics common to elderly subjects.

Referring to FIG. 1, the first variable resistor VCR₁ has a first inputterminal connected to the node N₁, a first output terminal connected toa third input terminal N_(A) of the first amplifying circuit OP₁, and acontrol terminal. The second variable resistor VCR₂ has a second inputterminal connected to the node N₂, a second output terminal connected toa fourth input terminal N_(B) of the first amplifying circuit OP₁, and acontrol terminal. The variable resistors VCR₁ and VCR₂ can be voltagecontrolled resistors whose values are varied according to the voltagelevel signals of the control terminals. In addition, after receivinginput signals through the variable resistors VCR₁ and VCR₂, the firstamplifying circuit OP₁ performs a first amplifying processing of theinput signals. Subsequently, the second amplifying circuit OP₂ connectedto the first amplifying circuit OP₁ amplifies the signals again toprovide to a post stage circuit for signal recording or analysis.

As shown in FIG. 1, the signal capture unit 14 is connected to the firstamplifying circuit OP₁. In this embodiment, the signal capture unit 14can be a bandpass filter configured to filter noise out of the bandsignals from the bio-potential signals, such as 50 Hertz, 60 Hertz, orother environmental noise signals. The logic control unit 16 isconnected to the signal capture unit 14 and to the control terminals ofthe variable resistors VCR₁ and VCR₂, and is configured to providecontrol signals S₁ and S₂ for adjusting the impedances of the variableresistors VCR₁ and VCR₂. In this embodiment, the logic control unit 16can be a microcontroller or a single chip processor.

FIG. 2 is a flowchart illustrating an exemplary embodiment of adifferential voltage sensing method, wherein the method is employed forachieving input impedance matching. In step S20, a first bio-potentialsignal is provided to a first variable resistor for generating a firstsignal. In step S22, a second bio-potential signal is provided to asecond variable resistor for generating a second signal. The first andsecond bio-potential signals have a common mode voltage level and adifferential mode voltage level. In step S24, the first and secondsignals are differentially amplified for generating a third signal. Instep S26, the operation band of the third signal is selected forgenerating first and second logic signals. In step S28, one of theimpedances of the first and second variable resistors is dynamicallyadjusted according to the first and second logic signals. The followingdescribes the details of the differential voltage sensing method of thepresent disclosure in conjunction with FIG. 1.

First, the first and second bio-potential signals are provided to thefirst and second variable resistors, respectively, via first and secondelectrode interfaces 11 and 12. The first and second variable resistorscan be resistors VCR₁ and VCR₂ as shown in FIG. 1. In one embodiment,when the bio-potential signals are ECG signals, the first electrodeinterface 11 is disposed on or adjacent to the left torso of a testsubject, while the second electrode interface 12 is disposed on oradjacent to the right torso of the test subject. As mentioned before,the effective impedances of the electrodes may be different so that theinput impedances of the first amplifying circuit OP₁ are unequal. Inthis case, the voltage amplitude of the output terminals N₃ willincrease and approach saturation. Therefore, one of the impedances ofthe first and second variable resistors VCR₁ and VCR₂, e.g., the firstvariable resistor VCR₁, is adjusted. After adjusting the impedance ofthe first variable resistor VCR₁, if the voltage amplitude of the outputterminals N₃ of the first amplifying circuit OP₁ begins to decline, thenthe impedance of the first variable resistor VCR₁ is continuouslyadjusted until the voltage amplitude of the output terminals N₃ reachesa minimum value. The minimum value represents that the differentialvoltage sensing system 10 has found the optimum input impedance matchingpoint. On the other hand, if adjusting the impedance of the resistorVCR₁ does not result in reduction of the voltage amplitude of the outputterminals N₃ of the first amplifying circuit OP₁, then the resistor VCR₁returns to its initial impedance, and the impedance of the resistor VCR₂is adjusted to reduce the voltage amplitude of the output terminals N₃.When the voltage amplitude of the output terminals N₃ reaches theminimum value, the differential voltage sensing system 10 has found theoptimum input impedance matching point.

Referring to FIG. 1, the adjustment of the impedances of the first andsecond variable resistors VCR₁ and VCR₂ is implemented via a logiccontrol unit 16. The logic control unit 16 sends signals for controllingthe impedances of the variable resistors VCR₁ and VCR₂ according to anoutput signal from a signal capture unit 14. In one embodiment, thevariable resistors VCR₁ and VCR₂ can be voltage controlled resistors,and the output signals of the logic control unit 16 can be voltage levelsignals. Therefore, the impedances of the variable resistors VCR₁ andVCR₂ can be increased by increasing the voltage levels of the signals S₁and S₂, or the impedances of the variable resistors VCR₁ and VCR₂ can bereduced by decreasing the voltage levels of the signals S₁ and S₂. Inanother embodiment, the impedances of the variable resistors VCR₁ andVCR₂ can be reduced by increasing the voltage levels of the signals S₁and S₂, or the impedances of the variable resistors VCR₁ and VCR₂ can beincreased by decreasing the voltage levels of the signals S₁ and S₂.

FIG. 3 is a block diagram of a differential voltage sensing system 30 inaccordance with another exemplary embodiment. Referring to FIG. 3, inwhich like elements of FIG. 1 are shown having like referencedesignations. Referring to FIG. 3, the differential voltage sensingsystem 30 comprises a common mode voltage level output unit 32 and adriven right leg (DRL) electrode 34 attached to the test subject's rightleg via a node N₄. The DRL electrode 34 is configured to reduce thecommon mode noise signal and improve common mode rejection ratio of thedifferential voltage sensing system 30.

FIG. 4 is a block diagram of a differential voltage sensing system 40 inaccordance with another exemplary embodiment. Like elements of FIG. 1shown in FIG. 4 having like reference designations. The differentialvoltage sensing system 40 attaches to the test subject's skin viamulti-electrodes or an electrode array. As shown in FIG. 4, multiplexers42 and 44 are connected to the variable resistors VCR₁ and VCR₂,respectively, and each multiplexer has a plurality of input terminalsconnected to a plurality of electrodes. The multiplexers 42 and 44 areconfigured to selectively connect one of the electrodes to the variableresistors VCR₁ and VCR₂ according to a scan signal. Subsequently, thelogic control unit 16 controls the impedances of the variable resistorsVCR₁ and VCR₂ according to output signals of the signal capture unit 14.The total impedances in the serial path between the input terminal N_(A)of the first amplifying circuit OP₁ and the corresponding electrode, andbetween the input terminal N_(B) of the first amplifying circuit OP₁ andthe corresponding electrode, are subsequently equalized by dynamicallyadjusting the impedances of the variable resistors VCR₁ and VCR₂.Accordingly, the common mode noise signals can be significantly reduced,thus protecting output signals from the effects of environmental noise,electrode material, and adhesion state of the electrode.

The above-described exemplary embodiments are intended to beillustrative of the disclosure principle only. Those skilled in the artmay devise numerous alternative embodiments without departing from thescope of the following claims.

1. A differential voltage sensing method for achieving input impedancematching, comprising the steps of: providing a first bio-potentialsignal to a first variable resistor for generating a first signal;providing a second bio-potential signal to a second variable resistorfor generating a second signal; differentially amplifying first andsecond signals for generating a third signal; selecting an operationband of the third signal for generating first and second logic signals;and dynamically adjusting one of the impedances of the first and secondvariable resistors according to the first and second logic signals;wherein each of the first and second bio-potential signals has a commonmode signal voltage level and a differential mode signal voltage level.2. The differential voltage sensing method of claim 1, wherein the firstand second bio-potential signals are electrocardiogram signals,electroencephalogram signals, electromyogram signals or electrooculogramsignals.
 3. The differential voltage sensing method of claim 1, furthercomprising amplifying the third signal to provide it to a post stagecircuit.
 4. The differential voltage sensing method of claim 1, furthercomprising providing the common mode signal voltage level of the thirdsignal to a driven electrode.
 5. The differential voltage sensing methodof claim 1, further comprising measuring an amplitude of the thirdsignal, wherein if adjusting one of the impedances of the first andsecond variable resistors cannot reduce the amplitude of the thirdsignal, then the other impedance of the first and second variableresistors is adjusted.
 6. The differential voltage sensing method ofclaim 1, wherein the first and second variable resistors are voltagecontrolled resistors, and the first and second logic signals are voltagelevel signals.
 7. The differential voltage sensing method of claim 1,further comprising selectively connecting two of a plurality of theelectrodes to the first and second variable resistors.